Structures in contact with seawater, for example ships, oceanic constructions, fish farming nets, buoys and industrial water systems, are constantly exposed to water being inhabiting by various marine organisms. Therefore, with the lapse of time, microorganisms such as bacteria and diatoms and, further, fouling organisms of larger size, for example such animals and plants as barnacles, mussels and sea lettuce, adhere to said structures. When the surfaces of the structures in contact with seawater are covered with such marine organisms, there take place corrosion of the covered part, decreased marine fuel efficiency due to increased frictional resistance of the ship bottom against seawater, massive death of fishes and shellfishes due to clogging of fish farming nets, decreased working efficiency, and sinking of buoys due to reduced buoyancy.
To prevent these hazardous organisms from adhering, antifouling coatings have so far been applied. Typical of such antifouling coatings are antifouling coatings of the hydrolyzable type which contain a trialkyltin-containing polymer as an essential antifouling component. These antifouling coatings produce their effects when the trialkyltin-containing polymer is hydrolyzed in the weakly alkaline environment in seawater, whereby the organotin compound is elated and at the same time the coating vehicle becomes soluble in water, allowing elution of the antifoulant incorporated. However, the organotin compound eluted is highly toxic and harmful to the ecosystem. Ecofriendly antifouling means have therefore been required from the environmental pollution viewpoint.
Silicone rubber coatings are well known to be capable of producing an antifouling effect without elution of an antifoulant and/or some other component. The silicone rubber coatings have both water-repelling property and elasticity, together with other properties. These properties are utilized to produce the antifouling effect. However, such coatings are attacked by microorganisms at a very early phase following submersion thereof in seawater, with the result that they lose water-repelling property and allow rapid progress of adhesion of marine organisms.
Various proposals have been made to maintain water repellency and attain an improved lasting antifouling effect. Thus, for instance, Japanese Kokai Publication Sho-53-79980 discloses a method which comprises incorporating a silicon- and metal-free organic compound, such as liquid paraffin, into a vulcanized silicone rubber. Japanese Kokoku Publication Sho-56-26272 discloses a method which comprises incorporating a silicone oil with a molecular weight of about 2,000 to 30,000 into a silicone rubber. Japanese Kokoku Publication Sho-60-3433 discloses a method which comprises adding a petroleum fraction-derived substance with a low critical surface tension, such as vaseline, to a silicone rubber. Japanese Kokai Publication Sho-54-26826 and Japanese Kokoku Publication Sho-57-16868 disclose a method which comprises incorporating various thermoplastic resins or a polyvinylbutyral resin into a silicone rubber. However, by these technologies, it is difficult to maintain the antifouling effect over a long period of time.
Japanese Kokai Publication Hei-07-328523 discloses a water-repelling coating film which comprises a resin coating and minute particles at least the surface of which is hydrophobic and which have a mean size of 1 nm to 1 mm and are fixed on at least 20% of the coating surface area. However, this technology still has a problem. Since the minute particles are caused to adhere to the resin in the step of coating formation when the resin is in the uncured or semicured state, the coating film, when used in water for a long period, allows the minute particles to come off from the coating, whereby the water repellency of the coating film is impaired.
Japanese Kokai Publication Hei-08-195126 discloses a technology for preventing the icing by a water-repellent coating film which has a minute unevenness on the surface and is coated on tops of the protrusions with a film formed by a water-repellent material. The coating film according to this technology has the protrusions with a height of 0.03 to 250 .mu.m at intervals of 0.1 to 50 .mu.m. Since this coating film, however, is formed by subjecting minute particles comprising, for example, an inorganic oxide to adhesion to the surface of coating layer using a means such as application and spraying, the coating film, when used for a long period, tends to allow the minute particles to come off from the coating, whereby the water repellency of the coating film is impaired. Moreover, since the unevenness is formed by subjecting the minute particles to application or spraying to the surface of coating layer, it is difficult to obtain a desired unevenness and a coating film having the minute protrusions with a desired size in a desired proportion to the surface area.
Japanese Kokai Publication Hei-08-26177 discloses a method of forming a surface decreasing a frictional resistance in water, which comprises mixing a specific powder with a resin and applying the resulting mixture to a surface of a structure to be coated. The surface of the structure to be coated according to this technology has the unevenness formed by the powder, the protrusions with a height of 0.09 to 90 .mu.m being at intervals of 0.1 to 30 .mu.m. Since this coating film, however, is formed by subjecting minute particles comprising, for example, an inorganic oxide to adhesion to the surface of coating layer using a means such as application and spraying, the coating film, when used in water for a long period, tends to allow the minute particles to come off from the coating, whereby the water repellency of the coating film is impaired. Moreover, since the unevenness is formed by subjecting the minute particles to application or spraying to the surface of coating layer, it is difficult to obtain a desired unevenness and a coating film having the minute protrusions with a desired size in a desired proportion to the surface area.
Furthermore, none of the technologies mentioned above have a suggestion nor a confirmation that the coating films can attain antifouling performances in water for a long period. Moreover, since the technologies needs a step of subjecting minute particles to adhesion, the formation of coating films is complicated and poor in practicality.
Accordingly, it is an object of the present invention to provide a nonelution type antifouling method by which the antifouling effect can be maintained for a prolonged period of time without adversely affecting the environment, a method of forming a nonelution type antifouling coating, and a nonelution type antifouling coating composition suited for said method.